This invention relates to wireless arrangements and, more particularly, to wireless signaling arrangements that employ packet transmissions.
In a conventional cellular network, a mobile unit communicates with a base station system (BSS) over a wireless link. The BSS communicates with a mobile switching center, which together with other mobile switching centers and other BSS comprises the cellular network. The cellular network is connected, in turn, to the public switched telephone network (PSTN).
In a system known as CDPD (Cellular Digital Packet Data), the mobile unit sends its information in packets. The BSS receives these packets and routes them to a data network that is separate from the cellular network. A current proposal for a system known as General Packet Radio Service (GPRS) contemplates having the mobile unit send packets to the BSS, and having the BSS forward the packets to an intelligent router. The router, in turn, sends the packets to the Internet or other packet data networks through a gateway router.
One system specification that is quite pervasive in many countries is GSM (Global System for Mobile communications). GSM employs 20 msec logical frames both in the uplink from mobile to the BSS and in the downlink from BSS to the mobile units. Each logical frame is divided into four physical frames. Each logical frame is also divided into 8 logical time slots. Likewise, each of the physical frames are also divided into 8 physical time slots. The uplink slots are coincident in time with downlink slots, and are numbered so that like-numbered slots (in the uplink and the downlink, respectively) are time shifted by three slots.
The proposed GPRS employs the GSM frame structure.
The GSM signaling specification includes uplink and downlink signaling channels that are used to set up sessions. To access the uplink signaling channel, for example in order to set up a session, mobile units employ random access contention techniques. The signaling channels are designed for circuit switched sessions, but they can also be used for sessions that transmit packets. An essential difference between circuit and packet sessions is that while a circuit involves a continuous activity, a packet session involves bursts of activity. Therefore, in a packet session, it is desirable to relinquish the channel during periods of inactivity and reacquire a channel for a new activity burst.
Alas, access via the call set-up channels is too slow for real time applications. The primary reason why this signaling channel is slow is that the protocol for setting up sessions involves a fair amount of computing and communication overhead. For example, the BSS needs to determine whether the load it is currently handling is low enough to admit a new session, the BSS needs to identify the mobile unit and contact the unit""s home base in order to determine the. unit""s bona fide, etc. Another reason why this signaling channel is slow is that the number of units that are contending for access on the signaling channel is large, and random access (contention) protocols inherently permit significant delays.
It is desirable to realize an arrangement which enables real time interactivity for packet switched sessions, while efficiently utilizing the available bandwidth. Such an arrangement can enable a variety of services to be offered with widely different Quality of Service requirements, such as packet voice, real-time audio/video, and other real-time applications.
An advance in the art is realized with an arrangement where, in addition to the control channels that are used to set up sessions, there is another, logically and physically separate and independent set of control channels that can be used as session control channels. Advantageously, such a session control channel is not within the session""s traffic-carrying channel and is available during both active sand inactive periods of a session.
In accordance with one illustrative embodiment, the physical GSM frame arrangement that employs 8 logical slots is used, with one slot both in the uplink and downlink dedicated to these separate control channels. In the downlink, such a control channel can be used to provide channel grant information that is broadcast to the relevant mobile, requests to make and transmit power measurements, poll the mobile units, etc. In the uplink, such a control channel can be used to request access to a traffic channel, to report on power measurements, to request special hand-off handling, to respond to polling, etc. Because the uplink session control channel does not need to carry a significant amount of information, the physical GSM slot can be divided into minislots, with each minislot constituting a session control channel for a group of mobile units. In accordance with one embodiment, a mobile unit chooses a particular session control channel and contends, on a random access basis, for access to a traffic channel. In another embodiment, the mobile units are assigned particular session control channels, on a dedicated basis. In still another embodiment, the BSS polls the mobile units via the downlink. When a mobile unit is polled, if it has data to be sent to the BSS, it does so.